The present invention relates generally to a microchip comprising a grayscale feature and a micromachined alignment feature and a process for creating such a microchip, and more specifically to a process for creating the grayscale feature in registration with the micromachined alignment feature from a single exposure mask.
Background of the Invention
Microchips having optical structures, such as lenses or gratings, often need to be accurately aligned with other discrete components, such as optical fibers, lasers, and photodetectors, as well as additional microchips. The desired alignments can be accomplished passively by using micromachined alignment features on the microchip to register the microchip with the other selected components. However, registration of the microchip with the discrete components does not necessarily ensure proper optical alignment between the optical structures on the microchip and their counterparts on the registered components. In order to facilitate the registration of the optical structures on the microchip with the optical structures on the registered components, the optical structures on the microchip need to be accurately aligned relative to the micromachined surface features on the microchip.
Typically, many desired optical structures are not binary in nature but are instead fabricated using a grayscale exposure mask to create the desired surface profiles. In contrast, however, a binary mask is particularly suited to the creation of micromachined alignment features. Alignment surface features are most reliable when they have clear demarcations of sufficient depth to permit precise alignment between discrete components. Therefore, a need exists for a process for creating a microchip having optical features, such as those created by a grayscale mask, in addition to micromachined surface features, created by a binary mask. While separate exposure masks may often be used, i.e. one mask for the grayscale features and a second mask for the binary features, the use of two exposure masks introduces the potential for mis-registration between the alignment features and optical features, due to misalignment between the two separate exposure masks. Therefore, it would be beneficial to develop a method for creating alignment features and grayscale features on a microchip from a single exposure mask.
In accordance with the present invention, a process is provided for creating a grayscale feature and a micromachined alignment feature within a substrate from a single exposure mask. The exposure mask includes a grayscale pattern representing the desired grayscale feature and an alignment pattern for defining the location of one or more micromachined features relative to the position of the grayscale pattern. The use of a single exposure mask ensures that the grayscale features and the alignment features are accurately aligned with respect to one another.
The process of the present invention includes a step of providing a protective layer over a first surface of a substrate at the intended location of the alignment feature. An unprotected portion of the first surface of the substrate is provided at the intended location of the grayscale features. The unprotected portion may be formed by removing a section of the protective layer at the desired location to form the unprotected portion of the first surface of the substrate. A photosensitive mask layer is then deposited over the protective layer and the unprotected portion of the first surface of the substrate.
The photosensitive mask layer is exposed to light of a selected wavelength through a single exposure mask. The light exposure functions to develop a replica of the selected grayscale pattern within the photosensitive mask layer at the unprotected portion of the first surface. The light exposure also functions to develop a replica of the selected alignment pattern within the photosensitive mask layer at the protected portion of the first surface. The portion of the photosensitive mask layer containing the replica of the selected alignment pattern is completely removed to produce an alignment aperture through the photosensitive mask corresponding to the intended location of the micromachined feature. Defining the location of the micromachined feature relative to the position of the grayscale feature using the single exposure mask functions to precisely align such features in the substrate to provide an alignment feature.
Next, a portion of the protective layer is removed from at least a region within the alignment aperture to create an alignment cavity in the protective layer. The alignment cavity functions to precisely marks the location of a selected micromachined feature relative to the substrate. The grayscale pattern recorded in the photosensitive mask layer is then transferred into the substrate to create the desired grayscale feature in the substrate. This transfer process also functions to remove any remaining portions of the photosensitive mask layer from the protective layer of the substrate.
A feature-protection layer is then deposited over the grayscale feature and over the protective layer including the cavity in the protective layer. A barrier layer, such as a photoresist layer, is then patterned over the feature-protection layer over the substrate. An opening in the barrier layer is provided over the cavity in the protective layer for access to a portion of the feature-protection layer. The barrier layer functions to protect the portion of the feature-protection layer in the vicinity of the grayscale feature during further processing. The exposed portion of the feature-protection layer within the barrier layer opening is removed to expose the alignment cavity. The barrier layer is then removed.
The remaining portion of the protective layer located at the base of the alignment cavity is then removed to expose the first surface of the substrate located at the base of the cavity. The exposed portion of the substrate at the base of the alignment cavity is then selectively removed to create the desired micromachined feature.
In a particular application of the process, the protective layer over the first surface of the substrate may comprise two layers, a first inner layer comprising SiO2 and a second outer layer comprising silicon nitride. In this configuration, the step of removing a portion of the protective layer to form the alignment cavity may comprise the step of substantially removing the outer silicon nitride layer to expose the inner layer at the base of the alignment cavity.
In addition, the step of creating a micromachined feature may include the step of anisotropic etching, which is particularly useful for creating micromachined features having sloped sidewalls, such as a V-shaped groove or pit. In this regard, the substrate material is chosen to have the proper crystal orientation for use with anisotropic etching to produce sloped sidewalls.
The invention also provides a microchip fabricated by the above process. The microchip comprises a substrate and a grayscale feature formed within a first surface of the substrate. A protective layer is disposed over the substrate. The protective layer includes an alignment feature aperture disposed at a selected positioned relative the grayscale feature. The microchip comprises an alignment feature disposed within the alignment feature aperture. The grayscale feature may comprise a refractive optical element or diffractive optical element. In particular the grayscale feature may comprise a lenslet array. The alignment feature may include a micromachined feature, such as a V-groove or pit. The microchip may optionally include a feature-protection layer disposed over the grayscale feature.